Polyimide has been widely used as an insulating film in electronic industries due to its flexibility, excellent insulation and heat resistance. With advancing technique in electronic industries, much more electronic elements use polyimide film as insulating film. Moreover, with acknowledge of environmental protection, many studies focus on improvement of properties of the polyimide.
In semiconductors industries, a circuit board is obtained by covering photo-resist film on a substrate, exposing the photo-resist to radiation through a mask having a desired pattern, developing and removing the exposed part or the unexposed part with an aqueous alkaline solution, then etching the substrate to obtain a circuit having a desired pattern. As to the photo-resist film, an aromatic polyimide has been widely used.
Such an aromatic polyimide such as photosensitive polyimide has been disclosed in U.S. Pat. No. 4,321,319 and U.S. Pat. No. 6,001,534. However, the developing for such aromatic polyimide should be done by using organic solvent so that it is unfriendly to environment and is harmful to operators' health.
Moreover, U.S. Pat. No. 5,668,248 disclosed a poly(amic acid) ester resin obtained by esterfying the carboxy group contained in the poly(amic acid) with a compound having no vinyl group. However, a film produced from the poly(amic acid) ester will result in film shrinkage in thickness direction when subjecting it to cyclization at an elevated temperature. Also, obtaining the starting monomer used in '248 patent requires a complex process and it is unfavorable to its production.
In view of environmental protection and the problems encountered in prior arts, there needs a polyimide resin which is water soluble and can be developed by using aqueous solution other than organic solvents.